Are you saying that until the message is not clicked on in the receiver's inbox, then it stays in the outbox? Because I could interpret "receive" as the message being written to whatever chunk of database represents the receiver's Inbox, regardless of whether they are even online, let alone read it. I don't know how the scripting works, it's not impossible that clicking on the send button does not send it immediately.
Correct. Unlike email, the message stays within the same system. There's no delay between pressing send and the message being sent - once the recipient opens the mail, it moves from your outbox to your sent items folder.
Given that it works differently than email, it's unfortunate that phpBB uses the same terminology, but it is what it is.